Homes with solar panels sold at a premium to comparable homes without solar systems over a nine-year time period, according to a new study by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory.

The average premium for a home with solar panels, also called a photovoltaic energy system, was about $17,000 for a relatively new, average-sized system rated for 3,100 watts.

The premium translates to an average of $5.50 per watt of solar power with the range of results stretching from $3.90 per watt up to $6.40 per watt.

California is approaching close to 100,000 individual solar systems installed. About 90 percent of them are residential.

The study examined data from more than 72,000 homes that sold in California from 2000 through mid-2009. About 2,000 of the homes had solar panels at the time of sale.

The researchers stated that they controlled for a number of other factors that can affect the home price, including housing market fluctuations, neighborhood effects, the age of the home, and the size of the home and the parcel on which it was located.

The study also found that the age of the solar system had an effect on the premium: older solar systems brought in less of a premium.

In addition, the data indicated that existing homes with solar systems enjoyed more of a premium than new homes with similarly sized solar systems.

“One reason for the disparity between existing and new homes with PV might be that new home builders also gain value from PV as a market differentiator that speeds the home sales process, a factor not analyzed in the Berkeley Lab study,” says Berkeley Lab researcher and co-author Peter Cappers. “More research is warranted to better understand these and related impacts.”